1. Field of the Invention
This invention relates to a photo-electric combined substrate having a photo-electric transducing function used in, e.g., optical communication, in which an optical waveguide is combined with an electric interconnection, and a manufacturing process therefor.
This invention also relates to an optical waveguide, in particular an optical waveguide with a higher heat resistance and a higher economic efficiency which can be manufactured by a relatively convenient process, and to a manufacturing process therefor.
2. Description of the Prior Art
Apparatuses such as an optical exchanger and a photo-interconnection device have been intensely investigated and developed for achieving large-capacity and high-speed communication. These apparatuses comprise an electric signal processing portion, an optical signal processing portion, and a transducing portion of an electric signal to an optical signal or vice versa. The transducing portion comprises a photoelectric transducing device (optical device) such as a laser diode (LD) and a photodiode (PD), and an electric element for operating the optical device or amplifying the signal.
In a conventional photo-interconnection, a silicon substrate is, in the light of its properties, used as a substrate on which an optical waveguide is formed and an optical device is mounted, while a ceramic substrate or printed board is frequently used as a substrate on which an electric interconnection is formed and an electric device is mounted. These substrates are mutually connected via a bonding wire in a manner that the substrate for the optical device is placed over the electric substrate.
In the conventional technique where the substrate on which an optical waveguide is formed and an optical device is mounted, is connected, via a bonding wire, with the substrate on which an electric interconnection is formed and an electric device is mounted, however, the wire is relatively longer. Therefore, when increasing an operating frequency for increasing a transmission capacity, a noise is overlaid on a signal. Thus, a higher frequency cannot be achieved.
In an attempt for solving the problem, several techniques have been proposed, in which both optical and electric devices are mounted on a single substrate; for example, JP-A 9-236731 has disclosed a ceramic substrate on which both optical and electric devices are mounted.
When both electric and optical devices are mounted on a ceramic substrate and these devices are closely mounted for high-speed operation of the optical device, the ceramic is responsible for heat insulation. However, a ceramic does not have a sufficiently low heat conductivity to prevent thermal interference between the electric and the optical devices.
On the other hand, when an optical device is mounted on a resin heat-insulating material on a ceramic substrate, the resin is so soft that an optical axis tends to be not in the right position.
For high-speed operation of an optical device, it is necessary to reduce the length of the electric interconnection between the electric and the optical devices. When the electric device and the optical device are mounted by separate methods, there is a restriction in reducing the length of the electric interconnection between the electric and the optical devices. There are also limitations in densification of, e.g., the electric and the optical devices.
A combined substrate described above has a configuration where an optical waveguide consisting of a siloxane polymer is formed on a ceramic multilayer interconnection substrate. Thus, when the interconnection substrate and the optical waveguide are made of different materials, it is necessary to form the electric insulating layer of the interconnection substrate and the optical waveguide with different materials by separate processes. It has been, therefore, difficult in the combined substrate to realize a complete three-dimensional combination or an adequately reduced cost for the optical waveguide and the electric interconnection. Furthermore, for a siloxane polymer used as a resin for an optical waveguide in the combined substrate, it is difficult to form a fine interconnection or a via-hole by photolithography process. The polymer cannot be, therefore, as a material for an electric insulating film.
In JP-A 3-245586, a semiconductor laser device is mounted on a resin such as a fluororesin (Teflon; trade mark) as an insulating material for preventing heat from being transferred from an electric device to the semiconductor laser device as an optical device.
Among others, an optical waveguide made of a resin has been intensely investigated and developed because it can be formed by a low-temperature and low-cost process into various types of substrates, leading to reduction in an overall cost for an optical module.
For example, F. Shimokawa et al., In Pr43rd ECTC (1993), p.705-710 and T. Matsuura et al., MES""97 (the Seventh Microelectronics Symposium), p.77-80 have disclosed an example where a fluorinated polyimide is used as a material for forming an optical waveguide.
According to these publications, the fluorinated polyimide is applied on a substrate and then heated to 300 to 400xc2x0 C. to form a film. Then, the optical waveguide core is processed into a desired shape by reactive ion etching. A copolymerization ratio between two polyimides can be varied to adjust a refractive index. The glass-transition temperature of the fluorinated polyimide is about 300xc2x0 C.
Furthermore, K. Enbutsu et al., MOC/GRIN ""97 Tech Digest, P3, p.394 and 1998 Electronic Information Communication Association Electronics Society Meeting Proceeding C-3-69 have disclosed an example where an ultraviolet (UV) curable resin (photosensitive epoxy resin) is used as a material for forming an optical waveguide.
As indicated in these publications, an ultraviolet curable resin has an advantage that only the core of the optical waveguide can be irradiated with UV to be cured into a desired shape. A main component in the UV curable resin can be varied to adjust a refractive index. The glass-transition temperature of the UV curable resin is about 250xc2x0 C.
In JP-A 10-170738, an optical waveguide is made of an asymmetric-spiro-ring containing epoxy acrylate resin.
However, when a fluorinated polyimide is used as a material for an optical waveguide, reactive ion etching is employed in forming the optical waveguide into a desired shape, leading to a longer etching duration, and thus it cannot be conveniently formed. In addition, available substrate materials are limited due to process factors such as a higher deposition temperature. Furthermore, a fluorinated polyimide has a disadvantage of a higher material cost.
On the other hand, an UV curable resin can be used to conveniently form a core shape because only UV irradiation is required. It is, however, used for a multi-mode optical waveguide whose core cross section has a width and a height of several ten micrometers because of an insufficient resolution for forming a fine pattern. Thus, it is not be applied to a single-mode optical waveguide whose core cross section has a width and a height of about several micrometers.
Furthermore, the UV curable resins as described in the prior art publications have a glass-transition temperature of about 250xc2x0 C., so that it cannot endure an optical device mounting step (about 300xc2x0 C.) using a gold/tin solder (melting point: 280xc2x0 C.) having a self-alignment effect, i.e., an effect that an optical device is drawn to a desired position by surface tension of a solder ball.
An object of this invention is to provide a photo-electric combined substrate in which an optical waveguide and an electric interconnection can be three-dimensionally combined with a reduced cost, and a manufacturing process therefor.
Another object of this invention is to provide a ceramic substrate comprising an optical device and an electric device which can operate with a high speed.
Another object of this invention is to provide a material for an optical waveguide which has good optical transparency at communication frequencies of 1.3 and 1.55 xcexcm and adequate heat resistance and can be conveniently shaped.
In the first aspect, it provides a photo-electric combined substrate comprising an electric interconnection part having an electric interconnection layer and an electric insulating layer as well as an optical waveguide part consisting of a core and a clad, where the electric insulating layer in the electric interconnection part and the optical waveguide part are made of the same material.
The photo-electric combined substrate of this aspect described above allows the electric interconnection part and the optical waveguide part to be formed by the same process, so that the optical waveguide part and the electric interconnection part can be three-dimensionally mounted and the photo-electric combined substrate can be prepared with a reduced cost.
The electric interconnection part and the optical waveguide part may be formed as separate structures.
Alternatively, the optical waveguide part may be placed on the electric interconnection part, or the optical waveguide part may be formed in the electric insulating layer of the electric interconnection part, which allows the photo-electric combined substrate to be further densified.
The material for the above-mentioned electric insulating layer and the optical waveguide may be a photosensitive resin whose refractive index depends on an exposure dose, and the core of the optical waveguide may be formed by scanning an exposure light while focusing on a desired position in the photosensitive resin such that the refractive index of the part to be the core of the photosensitive resin is higher than that of the part to be the clad of the photosensitive resin.
Moreover, the electric interconnection part and the optical waveguide part may be formed on the same substrate.
The above substrate may be a ceramic substrate, a single-layer interconnection substrate or a multilayer interconnection substrate.
This aspect also provides a process for manufacturing a photo-electric combined substrate comprising an electric interconnection part having an electric interconnection layer and an electric insulating layer as well as an optical waveguide part consisting of a core and a clad, which comprises steps of forming the electric interconnection part and forming the optical waveguide part, where the electric insulating layer in the electric interconnection part and the optical waveguide part are made of the same material.
It allows the electric interconnection part and the optical waveguide part to be formed by the same process. Furthermore, the optical waveguide part can be three-dimensionally mounted with the electric interconnection part, and the photo-electric combined substrate can be prepared with a reduced cost.
The steps of forming the electric interconnection part and forming the optical waveguide part may comprise the step of forming the electric interconnection part and the optical waveguide part as separate structures.
The step of forming the electric interconnection part and the optical waveguide part as separate structures may comprise the steps of depositing the material while separately forming the electric interconnection part and the optical waveguide part in given areas of the deposited material, and removing the deposited material where the electric interconnection part or the optical waveguide part is not to be formed.
The step of forming the optical waveguide part may comprise the step of forming the optical waveguide part on the electric interconnection part. Alternatively, in terms of the steps of forming the electric interconnection part and forming the optical waveguide part, the optical waveguide part may be formed in the electric insulating layer of the electric interconnection part during forming the electric interconnection part. It provides a further-densified photo-electric combined substrate.
The process of this aspect may comprise the step of forming the core of the optical waveguide part by scanning an exposure light while focusing on a desired position in the photosensitive resin such that the refractive index of the part to be the core of the photosensitive resin is higher than that of the part to be the clad of the photosensitive resin, using a photosensitive resin whose refractive index depends on an exposure dose.
The process of this aspect may comprise the step of forming the electric interconnection part and the optical waveguide part on the substrate and the substrate may be a ceramic substrate, a single-layer interconnection substrate or a multilayer interconnection substrate.
In the second aspect of the present invention, it provides a ceramic substrate having a concave where the concave is filled with a resin, and where at least an optical device is mounted on the ceramic substrate while an electric device on the resin in the substrate concave. Particularly, on the resin-filled concave, a fine electric interconnection layer using the resin as an electric insulating layer is formed.
This aspect also provides a process for manufacturing the above ceramic substrate comprising an optical device and an electric device, comprising the steps of forming a concave on a ceramic substrate; filling a resin in the concave; and mounting an optical device on the ceramic substrate while mounting an electric device on the resin filled in the concave.
An optical device, particularly a light emitting diode such as an LD significantly susceptible to heat may be mounted on a ceramic substrate while an electric device on a resin filled in a concave on the ceramic substrate. It can prevent thermal interference between the electric and the optical devices and thus it hardly brings about misalignment of an optical axis. Furthermore, the surfaces of the ceramic substrate and of the resin filled in the concave may be at the same level to reduce the wiring length between the electric and the optical devices and to further promote densification. In particular, a fine electric interconnection layer may be formed in the concave using the resin filled in the concave as an electric insulating layer and then an electric device may be mounted on the interconnection layer to meet the need of an electric device with a finer interconnection.
In the third aspect of the present invention, it provides an optical waveguide comprising a core and a clad having a refractive index lower than that of the core, where the core is made of a fluorene-unit-containing epoxy acrylate resin.
The clad is preferably made of a fluorene-unit-containing epoxy acrylate resin whose refractive index is lower than the material for the core.
The core or both of the core and the clad formed using the fluorene-unit-containing epoxy acrylate resin preferably have a glass-transition temperature of 260xc2x0 C. or higher, a light propagation loss of 0.5 dB/cm or less at a wavelength of 1.3 xcexcm, and a light propagation loss of 0.5 dB/cm or less at a wavelength of 1.55 xcexcm.
A particularly preferable fluorene-unit-containing epoxy acrylate resin described above is, for example, the following compound represented by formula (1): 
wherein X is the chemical structure represented by formula (2), Y is hydrogen or methyl, and n is an integer of 0 or more: 
wherein *s in the benzene rings indicate bonding positions to the chemical structure X in formula (1), and the positions * may be independently selected from ortho-, meta- and para-positions to the bonding position of the fluorene unit with the benzene rings; R1 to R16 are independently selected from a hydrogen atom, an alkyl group, an alkoxy group, an alkoxycarbonyl group, an aryl group and an aralkyl group.
This aspect of the present invention also provides a process for manufacturing an optical waveguide, comprising the steps of:
forming a lower clad layer on a substrate (Step 1);
forming a fluorene-unit-containing epoxy acrylate resin layer on the lower clad layer (Step 2);
exposing and etching the fluorene-unit-containing epoxy acrylate resin layer to form a core (Step 3); and
post-baking the substrate comprising the core (Step 4).
For forming a fluorene-unit-containing epoxy acrylate resin layer as the lower clad layer in the above Step 1, the fluorene-unit-containing epoxy acrylate resin layer is formed; then the resin layer is exposed with a dose more than that during the step of forming the core to control the refractive index of the lower clad layer to be lower than the refractive index of the core, and then the substrate comprising the lower clad layer is post-baked.
After the above Step 4, for forming a fluorene-unit-containing epoxy acrylate resin layer as an upper clad layer covering the core layer, the fluorene-unit-containing epoxy acrylate resin layer is formed; then the resin layer is exposed with a dose more than that during the step of forming the core to control the refractive index of the upper clad layer to be lower than the refractive index of the core, and then the substrate comprising the upper clad layer is post-baked.
The post baking may be conducted at a temperature of 160 to 250xc2x0 C.
The material for the optical waveguide according to the third aspect has the following features; first, it has a good optical transparency at communication wavelengths of 1.3 and 1.55 xcexcm; second, it has a glass-transition temperature of 260xc2x0 C. or higher, i.e., exhibits higher heat resistance; third, it is curable by UV irradiation owing to its epoxy acrylate groups, so that the core may have a cross section having a height and a width of several to several ten micrometers by UV exposure and development and thus the core may be readily formed; fourth, the refractive index of the fluorene-unit-containing epoxy acrylate resin may be controlled within an appropriate range by controlling an exposure dose, so that using the same material, a layer can be formed and then the clad and the core can be formed only by adjusting an exposure dose; and fifth, in contrast to a fluorinated polyimide, the film-formation of the resin requires a relatively lower temperature, resulting in a low-temperature process.
As described above, a fluorene-unit-containing epoxy acrylate resin can be used as a material for an optical waveguide to conveniently form a heat-resistant optical waveguide.